The wide spread use of data processing type equipment has led to an increased concern with the reliability of the power sources energizing this equipment. To protect the data processing load from the uncertainties of a single source of power, power supplies have been developed which have two separate sources of energy (AC line and battery) and the ability to draw power from one or the other so that the data processing load is continuously powered.
These uninterruptible power supplies have generally assumed either a serial or parallel architecture. In a serial architecture format, a rectifier, battery and inverter are connected in tandem with the battery floating when the AC source input to the rectifier is satisfactory. This arrangement is generally satisfactory from a consideration of a quality of a signal output to the load, but is inefficient since the overall efficiency can be no higher than the product of the efficiencies of the tandem connected components. In addition, failure of any of the tandem connected components may cause the entire power supply to fail.
The parallel architecture format avoids many of the disadvantages of the serial architecture format, since the two sources of energy are fully redundant with respect to each other. While efficiency and reliability is improved, the control arrangement for substituting the load from one power source to the other is more complex; especially in those applications where a load transfer must be transient free and transparent to the load. The control arrangement must maintain synchronism and a definite phase relation between the two redundant power processing paths. While these parallel architecture arrangements can be made cost effective at relative high power levels, the added control circuitry and complex transformer design requirements mitigate against their widespread use at relatively low power levels.